Markdown v.s. Pluto Notebook

[GiggleLiu]

I suggest using markdown instead of Pluto notebook as the solution file format.

1. Can be previewed on Github
2. VSCode support to markdown is very good:
   1. preview markdown files dynamically
   2. use copilot
3. You can render markdown file into many other formats. For example, we may consider generating pdf files from it.
4. Other people can contribute easier.

[ArrogantGao]

OK, I will use markdown for solution later.